The present invention relates generally to methods and apparatus for correctly registering a plurality of arrays of information upon or in a medium as at least one of the medium and means for imparting the arrays is moved in one direction with respect to the other.
More particularly, the present invention is directed to the provision of a registration apparatus and a method for multi-color electrostatic plotters.
Electrostatic plotters typically produce visible images on paper or film by imprinting an invisible electrostatic charge pattern on the paper, film or other medium and then making the latent image visible by developing it with an appropriate colored ink. The electrostatic image is a series of small dots impressed across the width of the medium at up to 400 dots per inch by an electrostatic image head which contains a plurality of electrodes which are appropriately charged (or not) to provide the desired pattern for the row of electrostatic information desired to be printed.
As is shown in the attached FIG. 1, such an image head, generally designated 10, may be provided with a linear array of spaced electrodes, including for each image row desired to be printed an odd row, generally designated 12, and an even row, generally designated 14, of electrodes which preferably are each 0.0027 inches in diameter spaced on 0.0050 inch centers between electrodes in a given even or odd row and thus, 0.0025 inch between adjacent dots in adjacent odd and even rows).
Electrostatic color plotters may be configured to produce a relatively large number of colors by sequential overlay of, typically, four separate color planes (see FIG. 2) in a similar manner as that used in offset printing of magazines. Combinations of yellow, magenta, cyan, and black inks may be effectively combined to produce a full spectrum of colors.
As is shown diagrammatically in side elevation in FIG. 2, such an electrostatic plotter, generally designated 20, includes a supply roll 22 from which a sheet of paper, film or other medium 24 on which an image is to be printed is unwound by the action of a drive roll 26 which is driven by appropriate control circuitry and mechanisms within the system control, generally designated 28. After leaving the drive roll 26, the paper is collected on a take-up roll 32 which is also controlled by the system control 28.
As the medium 24 is pulled from the supply ro11 22, it passes over an image head 34 with which it is kept in contact by a pressure roller 36 disposed between two guide or idler rollers 38. The image head 34 (which is configured as is shown for the image head 10 in FIG. 1) imparts charge pattern in a predetermined manner to the medium 24 in accordance with signals received from circuitry in the system control 28 and in accordance with plot data received via plot data lines shown as 40 corresponding to the raster color planes 42. The medium 24 so charged then comes into contact with the operative surface of one of a plurality of toning heads 44, 46, 48 and 50 for the application of toner of the appropriate color corresponding to the desired image pattern sought to be produced.
Each toner head, for example toner head 44, is associated with a reservoir of toner (not shown) which contains the appropriate color toner suspended in a liquid carrier, and similar reservoirs which are associated with each of the other toner heads contain similarly suspended toner for different ones of the primary colors used in the printing process. The appropriate toner is circulated by a pump (not shown) under the control of system control 28 through a conduit and the plurality of orifices (not shown) in a well-known manner to channels in the surface such as surface 44a of the toner head 44. The appropriate toner for the toner head flows over the toning surface of the head and then returns to its respective reservoir by way of the conduit and the channels in the toner head.
When the medium 24 passes over the appropriate toner head, it comes into contact with the carrier and the suspended toner, and the toners adhere selectively to charged portions of the medium 24 to form a color pattern corresponding to the charge pattern deposited by the electrodes on the image head 34.
Each of the toner heads 44, 46, 48 and 50 is provided with a vacuum conduit adjacent the right hand (as shown) portion thereof for removal of excess toner, and the plotter 20 is further provided (as is described in more detail in co-pending U.S. patent application No. 777,152, filed Sept. 18, 1985, in the name of Brian C. Preston, assigned to the Assignee of the present invention and entitled "Vacuum Toner Removal") with a separately actuable vacuum knife 52 which may be raised into position after the medium 24 is passed over the appropriate one of the toner heads 44, 46, 48 or 50 to remove any excess toner which may remain at or have migrated beyond the end of the plot. The details of said co-pending application are hereby incorporated herein by reference.
Such electrostatic color plotters use a multiple rewind method for the four color planes. Following the completion of each color plot, a clutch mechanism on the drive roll 26 (as is more fully described in U.S. patent application No. 756,547, filed July 19, 1985 now U.S. Pat. No. 4,621,535, in the names of Brian C. Preston and Bruce A. Bronson, assigned to the Assignee of the present invention and entitled "Forward Engagement and Reverse Disengagement Device") is disengaged, and the take-up roll 32, drive roll 26 and supply roll 22 are rewound so that the medium 24 may be returned to its initial position (by the system control 28) for successive passes for imaging of the additional color planes. The details of this latter application are hereby incorporated herein by reference. Such a multiple pass approach minimizes the number of image heads required, since each color plane uses the same image head with a different developer.
In the prior art, such as is disclosed in U.S. Pat. No. 4,500,045, issued to Whitaker et al on Feb. 19, 1985, registration of the color planes was accomplished with relatively complex, multiple mechanical servo-mechanisms to move the paper web medium 24 and/or the image head 34 for proper alignment of sequential color planes as they were being printed.
As is shown diagrammatically in FIG. 3, such prior art electrostatic color plotters registration mechanisms are provided with edge sensors 60 and 62 for substantially continually detecting the position of one edge (or a reference track, such as a longitudinal line generally in the longitudinal direction movement of the medium, and a plurality of shorter lines -"tracks"- spaced at predetermined intervals perpendicular to the longer longitudinal reference line) adjacent the supply roll 64 and the take-up roll 66, respectively.
The analog outputs of the edge sensors 60 and 62 are transmitted to amplifiers 68 and 70, respectively, and converted to digital signals in order to actuate step motors 72 and 74, respectively, which in turn physically laterally move the supply roll 64 and take-up roll 66, respectively, in accordance with movements of the edge of the medium or reference line detected by the edge sensors 60 and 62, respectively, in order to adjust for shifting or misalignment. The amount of movement of the edges or reference lines, the supply roll 64 and take-up roll 66 were fed back, respectively, via lines 76 and 78 to the edge sensors 60 and 62.
Similarly, prior art electrostatic color plotters, such as disclosed by Whitaker et al, were provided with a pair of line sensors 80 and 82 to detect the position of corresponding reference tracks on opposite sides of the medium and outputs corresponding to the positions of the tracks are transmitted to a differential amplifier 84 which converts the input signals from the sensor signals 80 and 82 into digital form, amplifying the difference and actuating a step motor 86 to move the image head 88 to compensate for differences in the reference tracks adjacent the image area in the direction of longitudinal movement caused, for example, by expansion or contraction of the paper or other medium as a result of stretching or of temperature or humidity changes during successive passes of the medium over the toner head. The position of at least one detected reference track is fed back via line 90 to the input of one of the line sensors 80 and 82.
Thus the prior art provided supply roll and take-up roll servo-mechanisms which kept the entering and exiting web or medium within coarse tracking. Precision alignment of the image planes was accomplished by the use of yet another servo-mechanism moving the image head to follow the reference tracks placed on the web during the printing of the first color plane. Each subsequent color plane was aligned to the initial plane by following the reference tracks placed on the medium during printing of the initial color plane.
Such a prior art electro-mechanical registration technique is mechanically and electronically complex and expensive, provides only relatively coarse adjustments for the longitudinal lateral displacement of the web during successive image passes and is affected by an extremely complex series of mechanical mechanisms which must constantly be kept in adjustment. In web offset presses, for example, accuracy can be achieved only through precision mechanical alignment of the printing elements and manual adjustment procedures at the start of each multicolor imaging process. Such techniques are not advantageously suited to plotters of the computer graphics type.
Accordingly, it is an object of the present invention to provide for correct registration of a plurality of patterns of information on or within a medium--for example, multicolor images on a paper web, in an extremely simple, reliable, easily attainable and efficient manner.
Another object of the present invention is to provide multiple color imaging with the correct registration of successive image planes imprinted on a medium by the use of relatively high speed, inexpensive electronic techniques, rather than the relatively complex, mechanical prior art mechanisms which require constant adjustment.